Technical Field
The present disclosure relates to a magnetoresistive sensor integrated in a chip for detecting magnetic fields perpendicular to the chip, as well as to its manufacturing process.
Detailed Description
In the following description, particular reference will be made to an anisotropic-magnetoresistor (AMR) sensor, without, however, being limited thereto, and the disclosure may be applied also to other types of magnetoresistive sensors, such as the giant-magnetoresistor (GMR) sensor and the tunneling-magnetoresistor (TMR) sensor and other integrated magnetic-field sensors that are sensitive to magnetic fields parallel to the chip integrating them.
As is known, magnetoresistive sensors exploit the capacity of appropriate ferromagnetic materials (referred to as “magnetoresistive materials”, for example the material known as “permalloy” which is an NiFe alloy) to modify their own resistance in presence of an external magnetic field.
Currently, magnetoresistive sensors are obtained from strips of magnetoresistive material. In case of anisotropic magnetoresistive sensors, during the manufacturing process, the strip of magnetoresistive material may be subjected to an external magnetic field so as to present a preferential magnetization in a pre-set direction (referred to as “easy axis”), for example the longitudinal direction of the strip itself.
Prior to measurement of the external magnetic field, an initial magnetization state along the preferential magnetization axis is set via a current pulse through a set/reset coil. In absence of external magnetic fields, the magnetization maintains the direction set by the set/reset pulse, and the strip has maximum resistance in this direction. In presence of external magnetic fields having a different direction from the preferential magnetization direction, the magnetization and resistance of the strip change, as explained hereinafter.
In FIG. 1, a magnetoresistor 1 is formed by a strip 2 of magnetoresistive material having a longitudinal direction parallel to the axis X, which also is the preferential magnetization direction EA. In absence of external magnetic fields, the magnetoresistor 1 is thus traversed by a current I flowing in the longitudinal direction of the strip. When the magnetoresistor 1 is immersed in an external magnetic field Hy directed parallel to axis Y (HA, “hard axis”), the magnetization M rotates through an angle α depending upon the amplitude of the external magnetic field Hy. In this condition, the resistivity of the magnetoresistor 1 changes. The change may be detected by an external circuit, which is thus able to determine the amplitude of the external magnetic field Hy.
Moreover, as shown in FIG. 2, in order to linearize the plot of the resistance R as a function of the angle α at least in an operating portion of the curve (magnetic fields of small dimensions), it is known to form, on the strip 2 of magnetoresistive material, a plurality of transverse strips 3 (referred to as “barber poles”), of conductive material (e.g., aluminum), arranged at a constant distance and with an inclination of 45° with respect to the direction X.
In this situation, the direction of the current I changes, but not the magnetization.
Generally, to simplifying reading, the magnetoresistors 10 are connected so as to form a Wheatstone bridge, as shown for example in FIG. 3, which shows a magnetoresistive sensor 11 designed to detect a magnetic field directed along axis X. In particular, the magnetoresistive sensor 11 comprises four magnetoresistors 10a, 10b forming two mutually parallel branches 4a, 4b, wherein the magnetoresistors designated by 10a have transverse strips 3 arranged at a first angle, here +45°, and the magnetoresistors designated by 10b have transverse strips 3 arranged at a second angle, here −45°. The branches 4a, 4b define input terminals 5, 6 and output terminals 7, 8. The magnetoresistors 10a, 10b of one branch are arranged specularly to the magnetoresistors 10b, 10a of the other branch so that the magnetoresistor 10a of the first branch 4a is connected to the second input terminal 6 and the magnetoresistor 10a of the second branch 4b is connected to the first input terminal 5. The magnetoresistors 10b are arranged in a similar way. Between the input terminals 5, 6 a biasing voltage Vb is applied.
In absence of an external magnetic field parallel to the detection direction (here the field Hx), the output voltage Vo between the output terminals 7, 8 is approximately zero. Instead, in case of initial magnetization directed vertically downwards, an external magnetic field Hx causes an increase in the resistivity of the magnetoresistors 10a and a corresponding reduction of resistivity of the other magnetoresistors 10b. Consequently, each resistance variation due to an external field perpendicular to the magnetoresistors 10a, 10b causes a corresponding linear variation of the output voltage Vo, the value whereof thus depends in a linear way upon the external magnetic field Hx.
When it is desired to detect both the components of a magnetic field parallel to the plane of the device integrated in the chip, it is possible to arrange the magnetoresistors 10 perpendicularly to each other, as shown in the sensor 12 of FIG. 4, which illustrates for simplicity a magnetoresistor 10x, for detecting the component X, and a magnetoresistor 10y, for detecting the component Y. Obviously, each magnetoresistor 10x, 10y of FIG. 4 may be replaced by a respective Wheatstone bridge similar to that of FIG. 3, wherein the four magnetoresistors 10x are oriented perpendicularly to the four magnetoresistors 10y. 
Because of the high sensitivity of the magnetoresistive sensors of the type indicated above, use thereof as electronic compasses in navigation systems has been proposed. In this case, the external field to be detected is the Earth's magnetic field. To a first approximation, the Earth's magnetic field may be considered parallel to the surface of the Earth, and compass reading may be made using the sensor 12, where X and Y represent the two directions of the plane locally tangential to the surface of the Earth. Since, however, the inclination of the compass with respect to the tangential plane entails reading errors, to correct the errors it is in practice common to have three sensors, each sensitive to a respective axis X, Y, Z.
To this end, the three sensors are perpendicular with respect to each other. Whereas creation of a sensor sensitive to fields oriented in two directions does not create difficulties, since they lie in a same plane, third direction detection is difficult, since prior art production methods construct a third sensor arranged in a plane perpendicular to the first two sensors.
Since current technologies do not enable industrial production at acceptable costs of vertical magnetoresistors, sensitive to the axis Z, in some compasses the sensor Z is obtained in planar form, as sensors X and Y, and the corresponding chip is attached on a base or frame in a vertical position, perpendicular to the sensors X and Y. However, in this case, the assembly operations are complex, and the end device is costly. Moreover, the packaged device has an excessive volume (in particular height), which does not enable use thereof in small apparatuses.
In order to solve this problem, it has been proposed to produce, alongside a planar magnetoresistor, a ferromagnetic concentrator transverse to the sensitivity plane of the magnetoresistor (see, for example, WO 2012/085296). For a better understanding, reference is made to FIG. 5, which shows a magnetoresistive sensor 15 formed in a chip 16 having a substrate 17 of conductive material, for example silicon, and an insulating layer 18. The substrate 17 has main surfaces 19, 20 parallel to the plane XY. The insulating layer 18 houses magnetoresistors 10z, which extend parallel to the main surfaces 19, 20 and to the plane XY and define the sensitivity plane of the sensor 15.
A concentrator 24 of “soft” ferromagnetic material (which may be magnetized easily and does not maintain magnetization after removal of the external magnetic field) extends in a trench 22 formed in the substrate 17 and extending throughout or almost throughout the thickness of the substrate 17. The concentrator 24 is here U-shaped and comprises an arm 24a having a length parallel to axis Z and much greater than its thickness. The arm 24a thus extends to near the main face 19 and is laterally offset with respect to the magnetoresistors 10z. 
Consequently, when the magnetoresistive sensor 15 is subjected to an external magnetic field Hz directed along axis Z, the arm 24a causes concentration and deflection of the field lines and generation of a horizontal field component Hy oriented along axis Y and thus parallel to the sensitivity plane. In particular, the flux lines deflected and concentrated traverse the magnetoresistors 10z, thus enabling detection of the external magnetic field Hz using a reading circuit in a known way.
In this known magnetoresistive sensor, however, it has been noted that, in some cases, because of the vertical distance (in direction Z) between the concentrator 24 and the magnetoresistors 10z and because of the inevitable misalignments in a horizontal direction (here in direction Y) that give rise to a gap also in this direction, the magnetic field deflected and concentrated in the concentrator 24 spreads out (spread phenomenon). This phenomenon causes an undesirable reduction and above all a sensitivity variation between sensors formed in different semiconductor wafers or even between sensors formed in a same wafer.